1) Field of the Invention
The present invention relates to a semiconductor laser device, a semiconductor laser module, and an optical fiber amplifier that realize a high output.
2) Description of the Related Art
In recent years, along the development of optical communications including the Internet or the like, optical fiber amplifiers are widely used in the middle of the transmission optical fiber, to transmit optical signals over a long distance. An optical fiber amplifier amplifies signal lights, whose intensity gets reduced during transmission, thereby to recover the intensity of the signal lights, in the course of the transmission of the signal lights through the optical fiber. Specifically, an optical fiber amplifier that uses an erbium-doped fiber (hereinafter referred to as an “EDF”) to amplify signal lights, and an optical fiber amplifier that utilizes a Raman amplification are proposed, and put into practical use. Both of these optical fiber amplifiers use a semiconductor laser device for a pumping source, and this semiconductor laser device excites signal lights. The amplification gain of each optical fiber amplifier increases together with the optical output from the pumping source. Therefore, the semiconductor laser device used for the pumping source needs to have high luminous efficiency and a high output. From this viewpoint, a buried-heterostructure laser (hereinafter referred to as a “BH laser”) as shown in FIG. 76 or the like is practically applied to the semiconductor laser device used for the pumping source.
The BH laser has a separate confinement heterostructure (SCH). As shown in FIG. 76, in the BH laser, a lower cladding layer 302, a lower SCH layer 303, a quantum well layer 304, and an upper SCH layer 305 are sequentially laminated on an n-type substrate 301. An upper portion of the lower cladding layer 302, and an active layer consisting of the lower SCH layer 303, the quantum well layer 304, and the upper SCH layer 305 sequentially laminated on the lower cladding layer 302, are processed into a mesa pattern. A p-type current blocking layer 307 and an n-type current blocking layer 308 are laminated adjacent to this mesa pattern on the lower cladding layer 302, thereby to form a current blocking layer. This current blocking layer has a function of shielding an injection current. Therefore, based on the existence of the current blocking layer, the BH laser contracts the injected current, thereby to improve the density of the carrier injected in the active layer, lower a threshold current value, and increase the luminous efficiency.
In recent years, there is an increasing requirement for obtaining a high output from the pumping source for the optical fiber amplifier, particularly, the pumping source for the Raman amplifier. Therefore, various investigations are carried out to obtain a high output laser beam emission from the semiconductor laser device.
To improve the optical output, it is necessary to increase the light guiding volume or capacity. To increase the light guiding capacity, it is considered necessary to increase the areas in the layer direction, the beam emission direction, and the horizontal direction respectively of the semiconductor laser device. The area in the layer direction is determined mainly based on the current confinement structure such as the SCH layer. Therefore, it is difficult to increase only the light confinement area in the layer direction separately from the current confinement. Next, the area in the beam emission direction is considered. A technique of increasing the waveguide area by increasing the length of the resonator is essential, and this method is employed in many cases. However, the increase in the length of the resonator is in the tradeoff relationship with the increase in the internal loss. Therefore, there is a limit to the improving of the optical output from the long resonator in a certain driving condition. Because of the reasons, to more improve the optical output, it is important to increase the waveguide area in the horizontal direction. When the waveguide area in the horizontal direction is increased, it is possible to decrease element resistance and thermal resistance. It is also possible to restrict saturation of the optical output due to heat. An upper limit of the area in the horizontal direction is determined based on a width Wc by which the waveguide mode in a high-order horizontal direction is cut off. A semiconductor laser device that has a width of at least Wc has a high-order horizontal direction waveguide mode, and loses a single peak in the emitted far-field pattern (FFP). Not only a kink occurs in the current and optical output characteristics, but also the coupling efficiency of the optical fiber is degraded extremely.
Referring to FIG. 3, a difference between an effective index of a first area 18 and an effective index of a second area 19 or a second area 20 is expressed as Δn. The width Wc is determined based on the lasing wavelength Δn. When the lasing wavelength Δn is smaller, it is possible to make the width Wc larger. Therefore, to control the width Wc, the control of the lasing wavelength Δn becomes necessary.
However, it is difficult in the BH laser to control the effective indexes. As a result, it is difficult to increase the area in the horizontal direction.
Reasons why it is difficult in the BH laser or other lasers to increase the light intensity distribution area in the horizontal direction are explained below.
The BH laser has the current blocking layer disposed adjacent to the active layer, as shown in FIG. 76. Usually, the active layer and the current blocking layer are constructed of mutually different semiconductor materials. The effective index of the first area that includes the active layer is determined based on the current confinement in the layer direction. A difference between the effective index of the first layer and the effective index of the second layer is determined based on the semiconductor material of the second layer that includes the current blocking layer. It is possible to control the refractive indexes based on only the selection of the materials as a parameter. Therefore, it is not suitable to carry out a fine control of the effective indexes based on the selection of the materials. The semiconductor material of the current blocking layer is determined based on the easiness of burring growth and the thermal resistance, and there is no degree of freedom in the selection of a material.
Taking a BH laser that uses an InP substrate, most of the BH lasers use InP for the current blocking layer. Among the semiconductor materials to be aligned in a lattice on the InP substrate, InP has a smallest refractive index. Therefore, there is a limit to a reduction in the difference between the effective indexes. Consequently, there is a limit to the increasing of the width Wc. As a result, the use of InP is not optimum to obtain a high output.
On the other hand, to minimize the difference between the effective indexes, the effective index of the first area that includes the active layer can be made smaller. However, in this case, the light confinement and the current confinement in the layer direction become weak. A carrier overflow becomes extreme at the time of injecting a high current, which hinders the obtaining of a high output.